Magnetic random access memories (MRAMs) using magnetic tunneling junctions (MTJs) are strong candidates to provide a dense (area=˜8-25 f2, where f is the smallest feature size), fast (1˜30 ns read/write speed), and non-volatile storage solution for future memory applications. The MTJ utilizes at least two magnetic layers that sandwich a thin dielectric insulting layer such as Al2O3, AlNxOy, or MgO, with one magnetic layer pinned by an anti-ferromagnetic film. To protect data from erasure or thermal agitation, an in-plane uniaxial magnetic anisotropy is needed for the magnetic free layer to store data.
As seen in FIG. 1a, the cross point of Word Line 11 and Bit Line 12 currents programs MTJ cell 13. The problem of keeping cells that share the same word line or bit line being disturbed is a major concern. Segmented Word Line approaches as described in “Segmented Write Line Architecture For Writing Magnetic Random Access Memories” [1] eliminate disturb conditions of cells on the same word line outside the selected segment. When the operating point is chosen deep along the hard axis, the required bi-directional bit line currents to program the selected cells are significantly reduced. The possibility of disturb along the bit line is also reduced. This is a most desirable MRAM operating condition.
Switching fields generated by word line and bit line currents for the conventional MRAM are about 30-60 Oe in intensity. For a segmented word line scheme, the bias field generated by the write word line is about 50-100Oe while the switching field generated by a bit line is about 15 Oe. The resulting shift of the uperating point is illustrated in FIG. 1b. For either case, the MRAM has to generate a relatively large magnetic field to rewrite recorder information so a relatively large electric current needs to flow through the address wirings.
As the device is further microminiaturized, the address wiring is reduced in width so that it becomes difficult to sustain the needed level of electric current. Furthermore, since the coercive force of the device is also increased, the current required to generate the necessary magnetic field, and hence the power consumption, is also increased.
For this reason, memory devices that use magnetization switching generated by spin transfer are receiving increased attention as an arrangement capable of switching the magnetization direction through application of a small electric current. In this scheme, reversal of the magnetic moment vector of the magnetic free layer is accomplished not by external magnetic fields, but by spin-polarized electrons passing perpendicularly through the stack of memory cell layers. A more detailed description is given in U.S. Pat. No. 5,695,864 by Slonczewski [2] and U.S. Pat. No. 6,532,164 by Redon et a. [3].
As described in the new concept outlined above, by sending an electric current through a magnetic layer having a particular magnetization, spins of electrons are oriented by quantum-mechanical magnetic exchange interaction with the result that the current carrying electrons leave the magnetic layer with a polarized spin. Alternatively, where spin-polarized electrons pass through a magnetic layer having a magnetic moment vector Hs in a preferred easy axis direction, these spin-polarized electrons will cause a continuous rotation of the magnetic moment vector. This may result in a reversal of the magnetic moment vector along its easy axis. Thus, switching of the magnetic moment vector between its two preferred directions along the easy axis may be effected by passing spin-polarized electrons perpendicularly through the magnetic layer.
Recent experimental data by Rippard et al. [4] confirm the very essence of magnetic moment transfer as a source of magnetic excitations and, subsequently, switching. These experiments confirm theoretical predictions by J. C. Sloncezwski [5] and J. Z. Sun [6]) stating that the spin-transfer generated net torque term acting on the magnetization under conditions of a spin-polarized DC current is proportional to:Γ=s nm×(ns×nm)where s is the spin-angular momentum deposition rate, ns is a unit vector whose direction is that of the initial spin direction of the current and nm is a unit vector whose direction is that of the free layer magnetization. The above equation indicates that the torque will be maximum when ns is orthogonal to nm.
Prior art that uses this torque is described in both U.S. Pat. No. 6,532,164 [3] and U.S. 2006/0092696 [7]. FIG. 2 is a schematic cross-sectional view of a storage element comprising a single memory cell of the prior art. This storage element consists of under-layer 21, anti-ferromagnetic layer 22, magnetization fixed reference layer 23, tunneling barrier layer 24, free layer 25, nonmagnetic spacer layer 26, magnetic drive layer 27 and capping layer 28. As the direction of magnetization M1 of the magnetization fixed layer 23 is fixed to point to the right hand side. This free layer stores information based on whether the direction of the magnetization M2 is to the right or to the left. Also, since the tunneling barrier layer 24 is located between the free layer 25 and the fixed reference layer 23, constituting an MTJ element, its magneto-resistance is used to determine the direction of the magnetization of the free layer 25.
The magnetization direction in the polarizing (or drive} layer is set to be perpendicular to the film plane. Maintaining magnetization in a direction normal to the plane of a disk is, in general, difficult to achieve. However, certain materials, such as TbFeCo or GdFeCo, used together with CoFe, at a total thickness of about 300 Angstroms, support this. The result is a high level of spin polarization of the FeCo. Magnetization of this type (i.e. normal to the main surface of the disk) are set up by exposing the disk to a magnetic field of at least about 2,000 Oe.
Electrons passing through the layers depicted in FIG. 2 are spin-polarized after passing through magnetic drive layer 27 by the effect of magnetic exchange interaction. Since the magnetization in drive layer 27 is perpendicular to the film plane, the polarization direction, ns, is also perpendicular to the plane of the magnetic free layer, resulting a torque on the free layer magnetization. When the current is large enough, it will overcome the anisotropy field in the free layer, resulting in a precessional movement within the film plane, causing its direction to switch back and forth at the precessional frequency. By controlling the current pulse duration precisely it is possible, in principle, to switch the free layer magnetization whenever required.
However, precession switching is a robust and fundamental effect. It is anticipated that maintaining such accuracy in the definition of the current pulse might prove to be extremely problematic, largely due to varying sources of impedance. Also the current density required for spin-torque switching is too large for ready integration with CMOS technology. Such a large current flowing across an MTJ tunneling barrier would generate long-term reliability problems.    1. U.S. Pat. No. 6,335,890 B1, W. R. Reohr, et al.    2. U.S. Pat. No. 5,695,864, J. C. Slonczewski    3. U.S. Pat. No. 6,532,164, Redon et al.    4. Phys. Rev. Lett. 92 (2004) 027201, W. H. Rippard et al.,    5. J. Magn. Magn. Mater. 159 (1996) LI, J. C. Sloncezwski,    6. Phys. Rev. B, Vol. 62, (2000) 570, J. Z. Sun    7. U.S. Pat. Pub. 2006/0092696 A1, K. Bessho
A routine search of the prior art was performed with the following additional references of interest being found:
U.S. Pat. Nos. 7,126,202 (Huai) and U.S. Pat. No. 7,088,609 (Valet) disclose magnetization switching generated by spin-transfer when a write current passes through a magnetic element. U.S. Pat. No. 7,006,375 (Covington) teaches a hybrid write mechanism using spin-transfer and using a half-select process. U.S. Pat. No. 6,980,469 (Kent et al.) discloses an MRAM device using spin transfer.
U.S. Pat. No. 6,771,534 (Stipe) discloses a spin-transfer method with thermal assist, U.S. Pat. No. 6,774,086 (Daughton et al.) shows another spin-transfer method, and U.S. patent Publication 2006/0087880 (Mancoff et al.) discloses spin-transfer with write lines. U.S. patent application No. 2005/0106810 (Pakala et al.) describes reduction of spin transfer switching current by the presence of a stress-assist layer.